1. Field of the Invention
The present invention relates to a frequency analysis method utilized in analyzing frequency components included in various kinds of signals and to a sweep type spectrum analyzer using this frequency analysis method, and more particularly, relates to a frequency analysis method which permits such spectrum analyzer to be swept at high or fast rates (speeds) even in a high resolution analysis and to a sweep type spectrum analyzer using such frequency analysis method.
2. Description of the Related Art
There have been two types of spectrum analyzers, one of which is referred to a sweep type spectrum analyzer and the other of which is an FFT (fast Fourier transform) type spectrum analyzer in correspondence to different methods of frequency analysis.
The sweep type spectrum analyzer means a spectrum analyzer of the type in which a local oscillator continuously performs a frequency sweep operation, a frequency spectrum component included in a signal to be measured is converted, by the frequency sweep operations, into an intermediate frequency signal consisting of a constant frequency component, and the power of the intermediate frequency signal is detected and displayed, as a spectrum component, on a screen of a cathode ray tube.
The FFT type spectrum analyzer means a spectrum analyzer of the type in which the oscillation frequency of a local oscillator is changed stepwise, the oscillation frequency in each step is resolved into a spectrum by the FFT transform means, and the Fourier transform results obtained in all of those steps are stored in a memory and are displayed on a display device.
The sweep type spectrum analyzer has a characteristic that all the frequency analysis results can be obtained by one frequency sweep operation. However on the other hand, there is a disadvantage in this type of spectrum analyzer that a time length required for one frequency analysis (time length of frequency sweep) must be longer as frequency resolution is made higher.
On the contrary, the time length required for a frequency analysis in the FFT type spectrum analyzer may be shorter than that required for the sweep type spectrum analyzer. However on the other hand, there is a disadvantage in this type of spectrum analyzer that since frequency analysis operation is performed stepwise, the frequency analysis results become discrete and hence all the spectrum components included in a signal to be measured cannot be extracted precisely.
As mentioned above, each of the sweep type spectrum analyzer and the FFT type spectrum analyzer has both merits and demerits. However, it can be said that if the sweep type spectrum analyzer can have a possibility of high rate sweep operations, the sweep type spectrum analyzer has a characteristic superior than that of the FFT type spectrum analyzer.
The reason why the sweep rates or speeds of the sweep type spectrum analyzer cannot be made higher is described in many technical books or journals (for example, "Spectrum Analyzer--Theory and Application" written by Morris Engelson and Fred Telewsky, translated by Kiyotaka Okada, and published by Nikkan Kogyo Shinbun Co., Ltd.; "Spectrum/Network Analyzer" written by Robert A. Witte, translated by Teruo Takeda and Nobutaka Arai, and published by Toppan Co., Ltd., and the like). Therefore, in this specification, such reason will be described very simply by concentrating on the items necessary for understanding the present invention.
First, a basic configuration of a sweep type spectrum analyzer will be described. FIG. 13 shows a configuration of a conventional sweep type spectrum analyzer in a mostly simplified form. As shown, the spectrum analyzer can basically be configured by a mixer 12, a local oscillator 13, an intermediate frequency filter 14, a sawtooth wave generator 15, and a display device 16. The mixer 12, the local oscillator 13 and the intermediate frequency filter 14 constitute, as will be mentioned later, a time-to-frequency converting apparatus 18.
The local oscillator 13 performs a frequency sweep over a preset frequency span or range f.sub.LO -f.sub.HI, and inputs the swept frequency signal LO to the mixer 12. The mixer 12 mixes the swept frequency signal inputted thereto from the local oscillator 13 and a signal to be measured S.sub.in inputted to an input terminal 11, and outputs, in this example, a difference signal between those two signals. Assuming that the center frequency of the passband of the intermediate frequency filter 14 is f.sub.IF, if the signal to be measured S.sub.in includes signals S.sub.1, S.sub.2 and S.sub.3 having frequencies f.sub.1, f.sub.2 and f.sub.3 (f.sub.1 &lt;f.sub.2 &lt;f.sub.3) respectively, intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 can be extracted through the intermediate frequency filter 14 every time the frequency f.sub.LO of the swept frequency signal LO satisfies conditions of f.sub.LO -f.sub.1 =f.sub.IF, f.sub.LO -f.sub.2 =f.sub.IF, and f.sub.LO -f.sub.3 =f.sub.IF, respectively.
By supplying the intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 extracted from the intermediate frequency filter 14 to a vertical input terminal Y of the display device 16, and by supplying a sawtooth wave signal S.sub.W outputted from the sawtooth wave generator 15 to a horizontal input terminal X of the display device 16, the intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 are displayed on the display device 16 the abscissa X of which is made a frequency axis, in order (sequence) of the frequencies f.sub.1, f.sub.2 and f.sub.3 (f.sub.1 &lt;f.sub.2 &lt;f.sub.3) respectively.
The example shown in FIG. 13 is a case in which the intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 extracted from the intermediate frequency filter 14 are directly inputted to a vertical input terminal Y of the display device 16. However, there is another case in which, as shown in FIG. 14, a detector 17 is disposed at the output side of the intermediate frequency filter 14, and the intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 are detected by the detector 17, and thereafter, this detected signal is supplied to the vertical input terminal Y of the display device 16 to display frequency spectrums S.sub.IF11, S.sub.IF12 and S.sub.IF13 having a rectified and smoothed single polar envelope.
In a practical case, since the bandwidth of the intermediate frequency filter 14 is narrow as compared with the frequency sweeping range (span) or swept frequency bandwidth of the local oscillator 13, the frequency spectrums S.sub.IF11, S.sub.IF12 and S.sub.IF13 are observed, as shown in FIG. 15, as line spectrums respectively if each of the signals S.sub.1, S.sub.2 and S.sub.3 included in the signal to be measured S.sub.in is a sine wave having single frequency.
From the above discussion, it could be understood that the signals S.sub.1, S.sub.2 and S.sub.3 included in the signal to be measured S.sub.in can be frequency-discriminated and can be converted, by the mixer 12, the local oscillator 13 for generating a frequency sweep signal and the intermediate frequency filter 14, to the intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 aligned on the time base in accordance with a lapse of time associated with the frequency sweep operation. Therefore, hereinafter, the frequency discriminating/converting means constituted by the mixer 12, the local oscillator 13 and the intermediate frequency filter 14 will be referred to as time-to-frequency converting apparatus or converter 18.
Here, attention is paid to the intermediate frequency signal S.sub.IF1 shown in FIG. 13. FIG. 16 shows a behavior in which the intermediate frequency filter 14 responds to a signal Smix.sub.1 having a difference frequency f.sub.LO -f.sub.1 outputted from the mixer 12.
Now, assuming that the center frequency of the intermediate frequency filter 14 is 10 MHz, the passband width of the intermediate frequency filter 14 defmed by -3 dB is .+-.1 MHz, the frequency f.sub.1 of the signal S.sub.1 is f.sub.1 =100 MHz, if the oscillation frequency f.sub.LO of the local oscillator 13 approaches 109 MHz at sufficiently slow speed, f.sub.LO -f.sub.1 =f.sub.IF is 109-100=9 MHz, and the frequency of the signal Smix.sub.1 falls into the passband width of the intermediate frequency filter 14. As a result, the intermediate frequency filter 14 starts to respond to the inputted signal Smix.sub.1 and to output at its output side an intermediate frequency signal B.sub.1 having 9 MHz frequency.
When f.sub.LO approaches 110 MHz, the difference frequency becomes f.sub.LO -f.sub.1 =10 MHz. Therefore, at this point in time, a signal outputted from the intermediate frequency filter 14 is a signal B.sub.2 having a frequency 10 MHz. Since the frequency 10 MHz of this signal B.sub.2 is equal to the center frequency of the intermediate frequency filter 14, the signal B.sub.2 has the maximum amplitude.
After the local oscillation frequency f.sub.LO exceeds 110 MHz, the amplitude of the intermediate frequency signal is gradually decreased. When the local oscillation frequency f.sub.LO approaches 111 MHz, the frequency of the signal Smix.sub.1 outputted from the intermediate frequency filter 14 approaches 11 MHz. The amplitude of the signal B.sub.3 outputted at this point in time is sufficiently small.
After the local oscillation frequency f.sub.LO exceeds 111 MHz, the intermediate frequency filter 14 gradually stops responding to the inputted signal Smix.sub.1 since the frequency of the signal Smix.sub.1 goes out of the passband of the intermediate frequency filter 14.
When the amplitude changes of the signals B.sub.1, B.sub.2 and B.sub.3 shown in FIG. 16B are connected or joined together, the intermediate frequency signal S.sub.IF1 shown in FIG. 16C is obtained. The amplitude of this intermediate S.sub.IF1 is proportional to the amplitude of the input signal S.sub.1. That is, the content ratios or percentages of the input signals S.sub.1, S.sub.2 and S.sub.3 are displayed as the amplitude ratios of the intermediate frequency signals. In addition, even if the frequencies f.sub.1, f.sub.2, and f.sub.3 of the input signals S.sub.1, S.sub.2 and S.sub.3 are mutually different frequencies, the intermediate frequency signal can always be extracted as a signal having a constant swept frequency bandwidth, in this example, having the swept frequencies ranging from 9 MHz to 11 MHz.
As previously noted, when the oscillation frequency f.sub.LO of the local oscillator 13 is swept at a sufficiently low speed, the envelopes A.sub.1, A.sub.2 and A.sub.3 (refer to FIG. 13) of the amplitudes of the intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 faithfully reproduce the filter characteristics of the intermediate frequency filter 14. Therefore, the correct spectrum frequencies and the power values of the respective spectrums can be displayed.
On the contrary, when the oscillation frequency f.sub.LO of the local oscillator 13 is swept at a high rate, as shown in FIG. 16D, the envelopes of the intermediate frequency signals S.sub.IF1, S.sub.IF2 and S.sub.IF3 produce two errors, i.e., a phenomenon that the peak frequency is shifted to the high frequency side from the center frequency of the intermediate frequency filter 14, and a drawback that the peak level is reduced. In addition, when the sweep rate is further increased, as shown in FIG. 16E, substantially the entire range of the swept frequency span (bandwidth) of the local oscillator 13 has a flat characteristic.
The explanation of the mechanism that causes the above two errors will be entrusted to the aforementioned various technical books, and the occurrence of such errors is the reason why the sweep rate or speed of the sweep type spectrum analyzer cannot be increased more and more.
Limit of the sweep rate of the sweep type spectrum analyzer generally spoken is defined as 0.5.times.RBW.sup.2. In this case, the RBW is a passband width of a filter determining a frequency resolution of a sweep type spectrum analyzer. Therefore, in the aforementioned example, the RBW corresponds to the passband width of the intermediate frequency filter 14. As is apparent from this definition, when the passband width RBW of the filter is made narrower in order to increase the resolution, the square value of the passband width RBW is decreased in inverse proportion. For example, when the passband width is 10 Hz, its square value is 100, when the passband width is 5 Hz, its square value is 25, and when the passband width is 1 Hz, its square value is 1. Therefore, there occurs a drawback that the sweep rate of the sweep type spectrum analyzer must be lowered in inverse proportion to the square of the passband width RBW of the filter as RBW is decreased.
There have been proposed various attempts for increasing more and more the sweep rate of the sweep type spectrum analyzer. As an example of those attempts, for example, there is an invention described in the Japanese Patent Application Laid Open No. Hei 4-221777 (221777/1992).
FIG. 17 shows a configuration of the spectrum analyzer described in the official gazette of the Japanese Patent Application. In FIG. 17, an external input terminal 11 is connected, similarly to the conventional spectrum analyzer, to a mixer 12 of a time-to-frequency converting apparatus 18 constituted by the mixer 12, a local oscillator 13 and an intermediate frequency filter 14. The intermediate frequency signals arranged along the time base by this time-to-frequency converting apparatus 18 are converted to digital signals by an AD (analog-to-digital) converter 19 and the digital signals are inputted to a quadrature detector 20, by which the digital signals are converted to complex signals. Each of the complex signals is inputted to a resolution filter 21, by which the complex signal is multiplied by the narrow band resolution filter characteristic through a convolution operation to determine the resolution. The power of the complex signal is detected through the resolution filter 21 and its spectrum components are stored in a memory 22. The spectrum components stored in the memory 22 are supplied to a display device 16 via a controller (CPU) 23 configured by a microcomputer, thereby displaying the spectrum components on the display device 16.
In addition, in the invention of the aforementioned Japanese Patent Application, the spectrum analyzer is constructed such that there is provided a calibration memory 24, by which the error that the spectrum power is reduced and the frequency shift error produced by sweeping at a high rate are compensated, and then the compensated spectrum is displayed on the display 16.
In the invention described in the Japanese Patent Application Laid Open No. Hei 4-221777, there is no teaching or suggestion at all that the power reduction error and the frequency shift error produced by sweeping at a high rate should be removed, and there is merely proposed a technical concept for compensating the already produced errors so as to apparently show the obtained data as if they were correct data. In the invention of the Japanese Patent Application, it is insisted that, as the result of high rate sweeping, the sweep rate comes to 2.266.times.RBW.sup.2, that is, "the sweep rate=2.266.times.RBW.sup.2 ". As compared with the conventionally defined sweep rate limit of 0.5.times.RBW.sup.2, this result of high rate sweeping (2.266.times.RBW.sup.2) is only approximately four times the conventional sweep rate of 0.5.times.RBW.sup.2, and the square term of the passband width RBW of the resolution filter remains still untouched. Therefore, it cannot be said that a true high rate or speed in sweeping has been attained.